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WEEKS 3WEEKS 3 -- 88

EMBRYONIC PERIODEMBRYONIC PERIODGROUP # 1

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INTRODUCTIONINTRODUCTION

Weeks 3 to 8 of human development is also known

as the embryonic period. During the embryonicperiod all major organs begin to develop and by

the end of this period the embryo has the distinct

appearance of a human.

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GASTRULATIONGASTRULATION

This is the development of the three germ layers

namely: ectoderm, endoderm, and mesoderm.

T

hese three layers are collectively referred to as thetrilaminar disk.

These three germ layers give rise to all adult tissue.

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y The first sign of gastrulation is the formation of the primitive streak

which is made of the primitive node, primitive groove, and primitive pit

(neur-enteric canal).

y The primitive streak is formed from epiblastic cells.

y Initially vaguely defined but in a 15 to 16 day embryo it is clearly

defined.

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y Epiblast cells then migrate towards primitive streak, become flask

shaped, detach from epiblast, and slip beneath it.This process is called

invagination.

y Cell migration and invagination is controlled by FGF8.

y Once cells invaginate some displace the hypoblast forming the

endoderm and some come to lie between the epiblast and newlyformed endoderm to form mesoderm.

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Gastrulation ContdGastrulation Contd

y More cells move between the epiblast and hypoblast causing them

the spread laterally and cranially.

y Some hypoblast cells towards the cephalic end of the embryo,

which are initially cuboidal become columnar in shape and fuse

with already columnar epiblast cells forming the buccopharyngeal

membrane (prochordal plate).

y This is the site of the future mouth.

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Notochord formationNotochord formation

y Prenotochordal cells invaginate and move cranially until they reach

the prechordal plate.

y These cells become intercalated in the hypoblast and for a short

time the midline of the embryo consists of two cell layers-notochordal plate.

y As the hypoblast is replaced by endoderm cells, notochordal plate

cells proliferate and detach from the hypoblast and form a solid

cord- definitive notochord.

y The notochord extends from the primitive node to the prechordal

plate

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Notochord FormationNotochord Formation

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MESODERMMESODERM

y Initially cells of the mesoderm form a thin

loosely woven tissue on each side of the

midline.

y The mesoderm give rise to:

o Paraxial mesoderm

o Intermediate mesoderm

o Later Plate mesoderm

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MesodermMesoderm

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Paraxial MesodermParaxial Mesoderm

y By the beginning of the third week the paraxial mesoderm

becomes segmented into somitomeres.

y They first appear in the cephalic region of the embryo and their

formation proceeds cephalocaudally.

y In the cepahlic region neuromeres are formed which contribute to

mesenchyme in the head.

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Paraxial Mesoderm ContdParaxial Mesoderm Contd

y Somitomeres then condense forming 42-44 pairs of

somites.

y Pair 1-7 contribute towards the formation of the

pharyngeal arches.

y T

he last 5-7 pairs of somites closest to the caudal endof the embryo disappear leaving 35 pairs of somites.

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Paraxial Mesoderm ContdParaxial Mesoderm Contd

y The 35 somites differentiate and lose the compact

orientation and form:

o Sclerotome - forms the cartilage, tendon and bone

components of the vertebral column.

o Myotome - forms epimeric and hypomeric muscles.

o Dermatome Dermis of the back.

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Paraxial MesodermParaxial Mesoderm

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Intermediate MesodermIntermediate Mesoderm

y Intermediate mesoderm which temporarily connects paraxial

mesoderm with the lateral plate and differentiates into the

urogenital structures.

y In cervical and upper thoracic regions forms segmental cell

clusters(Future nephrotomes), whereas more caudally it forms an

unsegmented mass of tissue, the nephrogenic cord.

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Intermediate MesodermIntermediate Mesoderm

ContdContd

y Excretory units of the urinary system and the

gonads develop from this partly segmented,

partly unsegmented mesoderm.

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Lateral Plate MesodermLateral Plate Mesoderm

y Lateral plate mesoderm splits into parietal and visceral

layers which line the intra-embryonic cavity and

surround organs respectively.

y Parietal layer mesoderm together with overlying

ectoderm will form the lateral and ventral body wall.

y The visceral layer and embryonic endoderm will form

the wall of the gut.

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Lateral Mesoderm ContdLateral Mesoderm Contd

y Mesoderm cells of the parietal layer surrounding intra-

embryonic cavity will form thin membranes, the

mesothelial membrane or serous membranes.

y These membranes will line the peritoneal, pleural and

pericardial cavities and secrete serous fluid.

y Visceral layer mesoderm form a thin serous membrane

around each organ.

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VasculogenesisVasculogenesis

y Blood vessels form in two ways:

oVasculogenesis whereby vessels arise

from blood island.

o Angiogenesiswhich entails sprouting

form existing vessels.

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Vasculogenesis ContdVasculogenesis Contd

y The first blood islands appear in the mesoderm of the yolk sac at

the 3rd week and slightly later in the lateral plate mesoderm and

other regions.

y Blood islands arise from mesoderm cells induced to formHeamangioblast. Heamangioblast are common precursors for blood

and blood vessel formation.

y The inducing factor is vascular endothelial growth factor or

(VEGF).

y Peripheral Heamangioblast differentiate into angioblasts.

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Vasculogenesis ContdVasculogenesis Contd

y The angioblast proliferate and are induced to form endothelial cells

by VEGF secreted from surrounding mesoderm cells.

y VEGF also regulates coalescence of endothelial cells into the firstprimitive blood vessels.

y Once the primary vascular bed is established additional vessels

form by angiogenesis also mediated by VEGF

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HeamopoiesisHeamopoiesis

y Heamangioblast in the center of blood islands form heamatopoietic

stem cells.

y The definitive haematopoietic stem cells arise from the mesoderm

surrounding the aorta.

y These cells colonize the liver, which becomes the major

haematopoietic organ of the fetus.

y Later stem cells from the liver will colonize bone marrow, thedefinitive blood forming tissue.

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EctodermEctoderm

y Notochord which lies below the ectoderm induces differentiation

of the ectoderm to form the neural plate.

y Once induction has occurred the neural plate expands, lateral

edges become elevated( neural folds), and the depressed regionforms the neural groove.

y Neural folds approach each other in the midline and fuse.

y The neural folds fuse forming the cranial and caudal neuropores.

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PROCESS OF NEURALATIONPROCESS OF NEURALATION

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EndodermEndoderm

y The gastro-intestinal tract is the main organ system derived fromthe endodermal germ layer.

y This germ layer covers the ventral surface of the embryo and formsthe roof of the yolk sac.

y The endoderm folds cephalo-caudally the anterior fold of theendoderm forms the foregut.

y The tail region forms the hindgut and the fold between the hindgutand foregut is the midgut.

Google Talk.lnk

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ReferenceReference

y Arnold SJ, Robertson EJ (2009). "Making a commitment: cell lineageallocation and axis patterning in the early mouse embryo".

y Gilbert, SF (2000). Developmental Biology (6th ed.). SinauerAssociates,

y Tam PL,Behringer, RR (1997). Human gastrulation: the formationof a mammalian body plan

y Antonio Nanci (2008). Ten Cate's oral histology: development, structure,

and function. Elsevier Health Sciences. pp. 25

y Evers,Christine A., Lisa Starr. Biology:Concepts and Applications. 6thed.United States:Thomson, 2006

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WEEKS 3 - 8